Obviously Bruz' treachery cannot go unanswered, but for the moment you have more pressing concerns. Ratbag tell you that your erstwhile captains are being held prisoner- it's your job to free your troops, and eliminate all the guards.
There will, of course, be an enemy captain keeping guard over the whole thing, but after what you've seen he's barely a concern, particularly once you start freeing captains of your own.
Once both objectives are met, you'll take an express cutscene straight to Bruz's throne room, along with the followers you just freed.
The two most dangerous elements of Bruz's army are the caragors and Bruz himself. Take out the former with Shadow Strikes, and don't turn your back on the latter. Otherwise it's a relatively easy fight, and most enemy attacks will be focused on your troops, allowing you free reign. As during the siege itself, it's preferable to convert enemies in order to expand your own forces.
Still, just when you have him on the ropes, Bruz will disappear, like a twelve foot tall ninja.
His judgment will be a task for another day, but in the meantime the fortress is yours again, (meaning you'll get to assign a new overlord, etc.) and your mission is complete.